You could start here, follow up there, read a book, search for related articles, plan your audit thoroughly, schedule a kick-off to check what the customer expects. Basically you'd want to review the hardware, operating system and installed firewall software and all related configuration, installed version, hotfixes, log files, best practices, licenses, contracts, updates, almost everything and create a proper documentation of your results. Check Point provides many tools you can use in addition to your own preparations, CheckMe, CPview, CPSizeMe, SmartEvent, Compliance Blade and so on. Maybe you'd want to conduct a Security Checkup as accompanying service as well (localizations available).

There are so many aspects when you say Technical audit. It may be for Security, Vulnerability, Risk etc.. So first need to check what exactly you need when you say technical audit. Well Below blades can be helpful for brief idea of activities.

You'll need to check within your Clients' Check Point UserCenter Account if the Appliance is covered by a valid Support Contract. Next thing you'll want to check is the Appliance's health by using the Check Point Appliance Hardware Diagnostic Tool. That should be everything your client wants to know at the moment.